1. Field of the Invention
A process for the conversion of hypophosphorous acid (H3PO2, HPA) and alcohols into various H-phosphonate diesters (RO)2P(O)H completely avoiding PCl3. Nickel chloride or nickel on silica catalyze the oxidative phosphorylation of alkyl phosphinates with various alcohols or water. The H-phosphonates so produced can be used in the preparation of various organophosphorous compounds including the industrially important herbicide glyphosate.
2. Description of the Prior Art
Organophosphorus compounds are very important compounds both economically and industrially. These compounds may be defined as compounds containing a phosphorus-carbon bond, although alkyl phosphate esters, which contain only phosphorus-oxygen (P—OR) bonds are the important exception to this definition. These compounds are used in numerous applications such as flame retardants, herbicides and pesticides, medicines and materials. One such class of compounds, the H-phosphonates are an important class of phosphorus-containing intermediates that are used in the production of one of the most prolific herbicide, glyphosate (N-phosphonomethylglycine). Glyphosate is sold commercially under the tradename Roundup® by Monsanto Company.
While the use of herbicides is not perfect, it would not be possible to sustain food production for the planet's population without them. Glyphosate has been widely used since its introduction in the 1970's and is considered to be “virtually ideal” due to its broad spectrum and low toxicity compared with other herbicides. Its use increased even more when Monsanto introduced glyphosate-resistant crops, enabling farmers to kill weeds without killing their crops. While herbicides are here to stay, more environmentally friendly and energy efficient production are essential for sustainable processes. Today, novel methods are needed to better employ feedstocks for organophosphorus synthesis and these new methods need to address many issues such as increased safety, efficiency and sustainability, lower energy consumption, and less waste product formation, to name a few.
Currently, almost all organophosphorus compounds are synthesized industrially from PCl3 which itself is prepared from the chlorination of elemental phosphorous (P4). The production of PCl3 is hazardous and environmentally problematic. Furthermore, PCl3 is a highly reactive compound and has been implicated in major industrial accidents. Additionally, the production of chlorine itself is even more dangerous and energy demanding as it is made via electrolysis. Some outdated chlorine plants are even responsible for a significant portion of preventable mercury pollution. The transformation of PCl3 into other organophosphorus compounds is never atom-economical and always results in the formation of wasteful HCl. Approximately half of all PCl3 produced globally goes into the production of the herbicide glyphosate.
Because of phosphorus trichloride's obvious drawbacks, significant research efforts have been devoted to bypass its use. A popular proposal relies on the so-called “P4-activation” pathway. Since P4 is already the precursor to all other organophosphorus compounds, as well as most inorganic phosphorus reagents, this appears to be a logical strategy. However, a fundamental problem with P4-activation is that not all phosphorus atoms can be used (the only exception being its conversion to PCl3). This is due to the fact that the phosphorus tetrahedron is broken, the reactivity of the P—P bonds decreases. Furthermore, P4 is toxic and pyrophoric, and thereby better employed in-situ. It is also insoluble in standard organic solvents (carbon disulfide is not a convenient solvent because of its extremely low flash point and autoignition temperatures).
Two other pathways that have received significant attention to bypass PCl3 in the preparation of organophosphorus compounds are: a) the use of PH3, and b) the use of Pred. Both have found some support in the literature and in practice. However, PH3 is a highly toxic and pyrophoric gas requiring very careful handling, and Pred uses “superbasic” conditions (aqueous KOH/DMSO, the Trofimov-Gusarova reaction) for functionalization. Additionally, the latter Pred-based approach requires heating white phosphorus to prepare Pred, and it does not solve the issue of phosphorus atom economy.
Approximately 350,000 metric tons of PCl3 are produced annually and as stated, 50% of PCl3 production goes solely into the production of glyphosate. It is expected that glyphosate usage will increase in the near future. Glyphosate is already nearly a $1 billion product. Because of the economic importance of glyphosate, there has been significant research devoted to its production. Generally, glyphosate is made from either one of three general processes: PCl3+formaldehyde+HN(CH2COOH)2 and then oxidative cleavage to glyphosate (HO)2P(O)CH2NCH2COOH, or a similar process using phosphorous acid (H3PO3). Phosphorous acid is currently made from the hydrolysis of PCl3 so 3HCl molecules are also formed. The third process, which is used in China, employs dialkyl H— phosphonates instead, but the current preparation of (RO)2P(O)H also produces 3HCl molecules from PCl3. These processes are shown in FIG. 1.
What is needed is a chlorine free route for producing organophosphorus intermediates such as H-phosphonates which could then be used to produce useful industrial-scale products, such as glyphosate. The ability to prepare glyphosate from phosphinates in this way circumvents the use of chlorine and reduces the production of harmful by-products. Although there is growing consensus that using chlorine and PCl3 should be minimized or avoided, the current method of production is unlikely to be easily changed due to the large scale of production and the complexity and costliness of changing large scale industrial processes. The present invention would represent a shift toward a more energy efficient, safer and environmentally friendlier production of glyphosate and is key to realigning the phosphorus economy away from PCl3.
The present invention has as its object, therefore, to develop a route for the catalytic transformation of phosphinates into H-phosphonates without the use of the hazardous compound PCl3, thereby enabling a more environmentally friendly synthetic route for the production of industrially important organophosphorus compounds, such as the herbicide glyphosate.